The acronym literally stands for:
High-Definition Multimedia Interface
It is an all digital system that carries both video and audio in digital form and has protocols that permit connected equipment to talk to each other.
The above is an advantage over existing forms of equipment interconnection.
But there are disadvantages.
The real reason for its existence is disabling rather than enabling.
Manufacturers and content producers were not prepared to unleash High Definition material onto the public unless they could control exactly what could be done with it.
The HDMI gives them that control. They can restrict what you watch , how many times you can watch it, control its quality and make copying impossible or only possible in reduced quality.
If any connected equipment does not comply they can make the signal degrade, be without sound , or produce no signal whatsoever.